Mirror elements for EUV lithography and production methods therefor

ABSTRACT

A method for the production of a mirror element ( 10 ) that has a reflective coating ( 10   a ) for the EUV wavelength range and a substrate ( 10   b ). The substrate ( 10   b ) is pre-compacted by hot isostatic pressing, and the reflective coating ( 10   a ) is applied to the pre-compacted substrate ( 10   b ). In the method, either the pre-compacting of the substrate ( 10   b ) is performed until a saturation value of the compaction of the substrate ( 10   b ) by long-term EUV irradiation is reached, or, for further compaction, the pre-compacted substrate ( 10   b ) is irradiated, preferably homogeneously, with ions ( 16 ) and/or with electrons in a surface region ( 15 ) in which the coating ( 10   a ) has been or will be applied. A mirror element ( 10 ) for the EUV wavelength range associated with the method has a substrate ( 10   b ) pre-compacted by hot isostatic pressing. Such a mirror element ( 10 ) is suitable to be provided in an EUV projection exposure system.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a Bypass Continuation of PCT/EP2010/068476, whichhas an International Filing Date of Nov. 30, 2010, and which claimspriority under 35 U.S.C. 119(a) to German Patent Application No. 10 2009055 119.0, filed on Dec. 22, 2009, the entire contents of which arehereby incorporated by reference in the disclosure of this application.

FIELD AND BACKGROUND OF THE INVENTION

The invention relates to production methods for mirror elements and tomirror elements comprising a reflective coating for the EUV wavelengthrange and a substrate.

It is known that the density of many materials, especially materialscontaining silicon, changes under irradiation with high-energyradiation. That effect is referred to in the literature as “compaction”.For applications in extreme environments (reactors, outer space) inparticular, studies were carried out and those effects werequantitatively determined a long time ago (see W. Primak, Nucl. Sci.Eng., 65, 141, 1978: “Radiation Behaviour of Vitreous Silica” and R. A.B. Devine, “Macroscopic and Microscopic Effects of Radiation inAmorphous SiO₂”, Nuclear Instruments and Methods in Physics Research B91 (1994), 378-390).

It has been found that the change in volume or density in silicondioxide typically attains, after sufficiently long irradiation, asaturation value on the order of about 2%-3% within the penetrationdepth reached by the radiation. The penetration depths of thehigh-energy types of radiation considered therein were typically in arange of from 0.5 μm to about 10 μm or more.

Comparable effects are also known in microlithography, especially forthe VUV wavelength range; however, owing to the relatively low level ofinteraction of the VUV light with the optical material used, especiallyquartz glass, the changes in volume in that case are as a rule in theppm range and therefore, typically, no saturation value is attained, theoptical materials used in that case being completely penetrated by theradiation.

A method is known from U.S. Pat. No. 6,205,818 B1 by which quartz glass(SiO₂) is to be made insensitive to compacting caused by long-termirradiation with UV laser radiation. The method provides for the quartzglass material to be pre-compacted by being exposed to high-energyradiation or by being pre-treated by hot isostatic pressing (HIP). Thehigh-energy radiation is said to make a compaction of between about 10ppm and 100 ppm possible, while a change in volume of the entire quartzglass body of from about 0.1% to about 3% is said to be achievable byhot isostatic pressing.

Since microlithography will have to rely in future on the EUV wavelengthrange in order to obtain a further increase in resolution and since,owing to their coating, the mirrors used in that case are capable ofreflecting only about 70% of the incident light and consequently absorbabout 30% of the incident light, materials having a low coefficient ofthermal expansion are normally used as substrate material for suchmirrors. Such so-called “low expansion materials” are, for example,Zerodur®, ULE® or Clearceram®. Those materials normally have anamorphous silicate glass content of above about 50% and, in extremecases, of even 100%. For a projection exposure system to be capable offunctioning on a long-term basis it is necessary to ensure, therefore,that the energy absorbed in the substrate material during operation doesnot lead to changes in the substrate and thus to degradation of themirror surface. In other words, it is necessary to ensure that changesof any kind in the shape or roughness of the surface, which can lead toa no longer tolerable increase in aberrations or stray light, do notoccur.

OBJECTS AND SUMMARY OF THE INVENTION

An object of the invention to provide production methods for mirrorelements so that the mirror elements exhibit no change or only anegligibly small change in surface shape on long-term irradiation withEUV. It is a further object of the invention to provide mirror elementsof that kind and an EUV projection exposure system for microlithographyhaving such mirror elements.

In accordance with one aspect, that object is attained by a method forthe production of a mirror element that has a reflective coating for theEUV wavelength range and a substrate, the method comprising:pre-compacting the substrate by hot isostatic pressing, and applying thereflective coating to the pre-compacted substrate, wherein either thepre-compacting of the substrate is performed until a saturation value ofthe compaction of the substrate by long-term EUV irradiation is reached,or, for further compaction, the pre-compacted substrate is irradiated,especially homogeneously, with ions and/or with electrons in a surfaceregion in which the coating has been or will be applied.

In accordance with the invention it is proposed that pre-compacting of amirror substrate of an EUV mirror be carried out by hot isostaticpressing. The penetration depth of the EUV radiation impinging on themirror element when in use, and hence the volume of the substrate inwhich a change in density may occur as a result of EUV irradiation, isadmittedly relatively small as a rule (typically some 100 nm). Theinventor has found that it is nevertheless advantageous for the entiresubstrate to be pre-compacted by hot isostatic pressing since such apressure treatment can be carried out relatively quickly andinexpensively in the case of the materials used as the substrate.

In one variant, there is selected as the substrate material a dopedglass material or a glass ceramic, which may be selected in particularfrom the group comprising: ULE®, Zerodur® and Clearceram®. Suchsubstrate materials have a low coefficient of thermal expansion whichmay, for example, be at most |0.5×10⁻⁷| 1/K in a range of 0° C. to 50°C. To produce such a low coefficient of thermal expansion (CTE), dopedglass or glass ceramic materials are typically used—for example, asmentioned above, ULE®, Clearceram® or Zerodur®. Glass ceramic materialshaving the low coefficient of thermal expansion indicated above consistas a rule of a crystalline phase and a glass phase. The crystallinephase has a negative coefficient of expansion which can precisely becompensated for by the positive coefficient of expansion of the glassphase. Glass materials having a low CTE are as a rule doped glasses, forexample TiO₂-doped quartz glass (ULE). It will be appreciated thatundoped glass, for example undoped quartz glass (fused silica), mayalternatively also serve as substrate material. The materials mentionedabove have an amorphous silicate glass content of more than about 50 wt.% and are therefore especially suitable for pre-compaction by hotisostatic pressing.

In the hot isostatic pressing, (initial and holding) temperatures ofbetween about 1100° C. and about 1300° C., preferably between 1150° C.and 1250° C., have proved to be especially favourable. It will beappreciated that it is not imperative for a single temperature to bemaintained during the hot isostatic pressing; rather, where appropriate,it is possible, for example, for cooling to take place from a maximumtemperature in several temperature stages.

In a further variant, the pressure in the hot isostatic pressing isselected to be between 20 MPa and 250 MPa, preferably between 50 MPa and150 MPa. The use of that pressure range has been found to be especiallyadvantageous for creating a high degree of pre-compaction.

In one variant, the holding time in the hot isostatic pressing isselected to be between 0.5 hour and 5 hours, preferably between 2 hoursand 4 hours. It has been found that a sufficient pre-compaction can beachieved even when relatively short holding times are used.

Especially in the case of materials having a high silicate glass contentof more than 90 wt. % (such as in the case of ULE®), the hot isostaticpressing may be carried out substantially as described in theabove-mentioned U.S. Pat. No. 6,205,818 B1, which regarding that aspectis incorporated into the present application by reference.

The substrate may be compacted by the hot isostatic pressing by at least1%, preferably by at least 1.5%, and in particular by at least 3%.Especially when the compaction is in the range of about 2%-3% or more,the saturation value of the compaction by long-term EUV irradiation canbe reached and therefore the substrate, once pre-compacted, cannot befurther compacted by the EUV radiation. It is, however, possible thatthe saturation value cannot be reached in the hot isostatic pressing orcan be reached only with excessively long holding times which may, forexample, be in the range of several days.

For that reason, for further compaction the pre-compacted substrate maybe irradiated, especially homogeneously, with ions and/or with electronsin a surface region in which the coating has been or will be applied.With this irradiation, a surface region extending from the surface ofthe substrate over a small depth, typically in the range of severalmicrometers, can be additionally compacted, so that the saturation valueof the density change is reached at least in that region. In this case,the pre-compaction using the inexpensive hot isostatic pressing processpermits the irradiation times of the ion or electron irradiation to beconsiderably reduced and thereby allows any form change possiblyoccurring as a result of the ion or electron irradiation to be kept assmall as possible.

The irradiation is advantageously carried out with high-energy ionshaving an energy of between 0.2 MeV and 10 MeV at a total particledensity of from 10¹⁴ to 10¹⁶ ions per cm² and/or with high-energyelectrons having a dose of between 10 J/mm² and 2000 J/mm² at energiesof between 10 KeV and 20 KeV. The irradiation may be carried out in thiscase especially as described in the Applicant's U.S. 61/234,815, whichis incorporated in this application by reference. It will be appreciatedthat, before and/or after the irradiation, additional processing steps,especially smoothing steps at the surface of the substrate, may becarried out, for example as described in the Applicant's U.S.61/234,815.

In one variant, the irradiation is carried out until there is obtainedin the surface region a density that is at least 0.5%, preferably atleast 1%, in particular at least 1.5% higher than the density of theremainder of the substrate. Together with the change in density obtainedon pre-compaction of the substrate it is possible in this case for thesaturation value of the compaction to be attained in an especiallysimple manner. The additionally compacted surface region extends as arule in this case to a depth of about 5 μm from the surface of thesubstrate, the exact value depending on the ion or electron energy whichis typically so selected that the compacted surface region extends asleast as far as the penetration depth of the EUV radiation on use of themirror.

A further aspect of the invention relates to a mirror elementcomprising: a reflective coating for the EUV wavelength range, and asubstrate, wherein the substrate is pre-compacted by hot isostaticpressing. Either the entire substrate is pre-compacted to a saturationvalue of the compaction of the substrate by long-term EUV irradiation,or a surface region of the substrate that extends beneath the coatinghas a density that is at least 0.5%, preferably at least 1%, inparticular at least 1.5%, higher than that of the remainder of thesubstrate. The density of the pre-compacted substrate material markedlyexceeds the density of the substrate material attained in a conventionalproduction process (without pre-compaction).

As explained above, the material of the substrate is typically a dopedglass material or a glass ceramic having a low coefficient of thermalexpansion, especially ULE®, Zerodur® or Clearceram®. Apart from the lowthermal expansion of those substrate materials they have the additionaladvantage that they have a high silicate glass content (about 50 wt. %or more). With that material, a considerable degree of pre-compactioncan be achieved by hot isostatic pressing with relatively short holdingtimes.

In one embodiment, the material of the substrate is a quartz glass dopedwith TiO₂, in particular ULE®, an initial density of the substratebefore compaction being 2.21 g/cm³. That initial density is typicallyobtained in ULE® that has been produced by a conventional productionprocess.

Especially when the saturation value of the density change is not yetreached by hot isostatic pressing, the resistance of the substrate toEUV irradiation can be increased if a surface region of the substrateextends beneath the coating, which surface region has a density that isat least 0.5% higher, preferably at least 1% higher, in particular atleast 1.5% higher than that of the remainder of the substrate and whichsurface region has been obtained through high-energy ion or electronirradiation. As described above, it is advantageous if that surfaceregion extends to a depth of about 5 μm from the surface of thesubstrate. By homogeneous irradiation with ions and/or electrons it ispossible to achieve a homogeneous compaction of the substrate in thatsurface region.

A further aspect of the invention is implemented in an EUV projectionexposure system for microlithography, comprising an illumination systemand a projection system having at least one mirror element for the EUVwavelength range as described above. In such an EUV projection exposuresystem, the surface shape of the mirror elements designed as describedabove changes on EUV irradiation only negligibly during the useful lifeof the system, and therefore no appreciable surface deformations thatmight lead to an increase in aberrations or stray light occur anylonger.

Further features and advantages of the invention will be apparent fromthe following description of illustrative embodiments of the inventionwith reference to the Figures of the drawings, which show detailsessential to the invention, and from the claims. The individual featuresmay be implemented individually or a plurality thereof may beimplemented in any desired combination in a variant of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

Illustrative embodiments are shown in the schematic drawings andexplained in detail in the following description. In the drawings:

FIG. 1 is a schematic illustration of an embodiment of an EUVlithography system having a plurality of mirror elements,

FIG. 2 is a schematic illustration of a pressure chamber for the hotisostatic pressing for pre-compacting a mirror substrate, and

FIG. 3 is an illustration of a mirror element for the EUV projectionexposure system of FIG. 1 with the substrate of FIG. 2.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

FIG. 1 shows schematically an EUV projection exposure system 1comprising a beam-shaping system 2, an illumination system 3 and aprojection system 4 which are accommodated in separate vacuum housingsand are arranged in succession in a beam path 6 from an EUV light source5 of the beam-shaping system 2. A plasma source or a synchrotron, forexample, may be used as the EUV light source 5. The emitted radiation inthe wavelength range of between about 5 nm and about 20 nm is firstcollimated in a collimator 7. Using a downstream monochromator 8, thedesired operating wavelength is filtered out by varying the angle ofincidence, as indicated by the double-headed arrow. In the wavelengthrange mentioned, the collimator 7 and the monochromator 8 are usually inthe form of reflective optical elements, with at least the monochromator8 not having a multilayer system on its optical surface in order toreflect a wavelength range as broad as possible.

The radiation treated in the beam-shaping system 2 in respect ofwavelength and spatial distribution is introduced into the illuminationsystem 3 which has a first and second mirror element 9, 10. The twomirror elements 9, 10 pass the radiation onto a photomask 11 forming afurther mirror element that has a structure which is projected on areduced scale onto a wafer 12 by the projection system 4. For thatpurpose, a third and fourth mirror element 13, 14 are provided in theprojection system 4. The mirror elements 9 to 14 are in this casedisposed in the beam path 6 of the EUV projection exposure system 1 andtherefore are exposed to long-term EUV irradiation.

As illustrated in FIG. 3 for the example of the second mirror element 10of the illumination system 2, that mirror element consists of areflective coating 10 a applied to a substrate 10 b. The reflectivecoating 10 a is a multilayer system having alternating layers ofmolybdenum and silicon whose thicknesses are coordinated in such amanner that as high a reflectivity as possible is obtained at theoperating wavelength of the EUV projection exposure system 1 of about13.5 nm. To avoid deformation on heating of the mirror element 10, thesubstrate 10 b consists of a material having a low coefficient ofthermal expansion, typically doped glass or a glass ceramic, for exampleULE®, Zerodur® or Clearceram®. Those materials have a silicate contentof more than about 50 wt. %, the density of which becomes greater on EUVirradiation of the substrate 10 b, in which case the surface of themirror element 10 may deform, which may lead to aberrations andincreased formation of stray light.

To avoid compacting of the substrate 10 b on long-term irradiation withthe light of the EUV light source 5, a pre-compaction of the substrate10 b is carried out by placing it in a hot isostatic press 20 which isshown in FIG. 2. The press 20 is operated in this case in an inert gasatmosphere 20 a, for example with argon being used as the inert gas. Inthe case under consideration, a sample of the substrate 10 b was kept ata temperature T of about 1200° C. and at a pressure p of about 100 MPafor a period of about 4 hours, with both the heating-up and thecooling-down being performed at a rate of about 10 K/min. If, as in theexample under consideration, ULE®, i.e. TiO₂-doped quartz glass, havinga titanium dioxide content of about 8 wt % is used as the material forthe substrate 10 b, it can be compacted in the hot isostatic pressingoperation using the above-mentioned parameters by about 1.5%-2% relativeto its initial density of about 2.21 g/cm³.

To verify the degree of compaction obtained by the hot isostaticpressing, the ULE® sample was subjected to electron irradiation at anenergy dose of about 3.7×10¹¹ rad to simulate long-term EUV irradiation.The form change obtained in that case was about 5 nm, and the degree ofcompaction, determined by interferometer measurement, was about 0.45%.By comparison, in the case of a non-pre-compacted ULE® sample exposed toelectron irradiation at a comparable dose (4.1×10¹¹ rad), a degree ofcompaction of about 1.82% was measured, with the form change being about20 nm. Using the hot isostatic pressing, therefore, it was possible toachieve a reduction in compaction to a quarter of the value obtained ina sample that had not been pre-treated. In the case of ULE® orcomparable materials, the maximum compaction (saturation value) underelectron irradiation occurred as a rule above a dose of about 4 to6×10¹¹ rad.

In silicon dioxide—and in ULE® glass also—the saturation value of thedensity change after long-term irradiation with EUV radiation islikewise typically on the order of about 2%-3% within the penetrationdepth reached by the radiation, which is typically not more than 5 μm.That saturation value may not quite be achieved by pre-compaction of thesubstrate 10 b using the above parameter values.

In order nevertheless to prevent the substrate 10 b of the mirrorelement 10 from becoming further compacted on EUV irradiation, it ishomogeneously irradiated with high-energy ions 16 in a surface region 15in which the coating 10 a has been applied. The ions 16 generally havein this case an energy of between about 0.2 MeV and about 10 MeV at atotal particle density of from 10¹⁴ to 10¹⁶ ions per cm². Alternatively,the substrate 10 b may also be irradiated with high-energy electrons,the dose typically being in this case between about 10 J/mm² and 1000J/mm².

The irradiation with ions or electrons is carried out in this case untilthe density in the surface region 15 is at least 0.5%, and whereapplicable at least 1% or 1.5%, higher than the density of the remainderof the substrate 10 b. The surface region 15 typically extends in thiscase at least as far as the penetration depth of the EUV radiation intothe substrate 10 b, which in the present case is about 5 μm. Bycombining the hot isostatic pressing with the irradiation, therefore, itis altogether possible for the saturation value for the density changeto be attained at least in the surface region 15, so that even onlong-term irradiation with the light of the EUV light source 5 thesubstrate 10 b may experience only extremely slight surface deformationdue to the change in density of the substrate 10 b.

It will be appreciated that the irradiation with ions and/or electronsmay also be performed on the substrate 10 b before coating, and thatfurther process steps, especially smoothing of the substrate surface,may be carried out between the hot isostatic pressing and theapplication of the coating. For details of the irradiation of thesubstrate with ions or electrons, reference is again made to theApplicant's U.S. 61/234,815.

It is again explicitly mentioned that, on suitable selection of theparameters of the hot isostatic pressing, where applicable alsoexclusively by pre-compaction of the substrate 10 b, an increase indensity of about 1%-3% can be achieved which, depending on the substratematerial used, corresponds to the saturation value of the densitychange, and therefore the subsequent irradiation with ions or electronsmay, where applicable, be dispensed with. In each case, mirror elementswhose density remains unchanged under long-term irradiation with EUVradiation are obtained in the manner described above.

The above description of the preferred embodiments has been given by wayof example. From the disclosure given, those skilled in the art will notonly understand the present invention and its attendant advantages, butwill also find apparent various changes and modifications to thestructures and methods disclosed. The applicant seeks, therefore, tocover all such changes and modifications as fall within the spirit andscope of the invention, as defined by the appended claims, andequivalents thereof.

The invention claimed is:
 1. A mirror element comprising: a reflectivecoating for extreme ultraviolet wavelength radiation, and a substratehaving a surface region, wherein: the surface region extends to a depthof no more than 5 μm from the surface of the substrate, the substrate ispre-compacted by hot isostatic pressing, and the surface region of thesubstrate that extends beneath the coating has a density that is atleast 0.5% higher than a density of a remainder of the substrate that isnot in the surface region.
 2. The mirror element according to claim 1,wherein the material of the substrate is a doped glass material or aglass ceramic.
 3. The mirror element according to claim 2, wherein thematerial of the substrate is a quartz glass doped with TiO₂, the initialdensity of the substrate before compaction being 2.21 g/cm³.
 4. Aprojection exposure system for microlithography at extreme ultraviolet(EUV) wavelengths, comprising an illumination system and a projectionsystem having at least one mirror element for the EUV wavelengths inaccordance with claim
 1. 5. The mirror element according to claim 1,wherein the surface region of the substrate that extends beneath thecoating has a density that is at least 1.0% higher than a density of theremainder of the substrate that is not in the surface region.
 6. Themirror element according to claim 1, wherein the surface region of thesubstrate that extends beneath the coating has a density that is atleast 1.5% higher than a density of the remainder of the substrate thatis not in the surface region.